1. Field of the Invention
The present invention relates to a technique for removing material from the perimeter of a substrate.
2. Description of the Prior Art
In the production of semiconductor devices, it is frequently desirable to grow an epitaxial layer of material on a single-crystal semiconductor substrate. For example, silicon, germanium, or III--V semiconductor materials are epitaxially grown on silicon wafers. It is known to form a "cap" of material on the back-side of a wafer prior to the epitaxial growth on the front side of the water, in order to prevent autodoping. That is, the cap, typically of silicon dioxide, silicon nitride, or both, prevents dopants in the wafer from out-diffusing from the back side of the wafer and inadvertently doping the epitaxial material on the front side. One method of forming a silicon dioxide/silicon nitride cap on a wafer is disclosed in U.S. Pat. No. 4,687,682 coassigned with the present invention, with other techniques also known in the art. To a large extent, the cap also prevents the silicon (or other epitaxially deposited semiconductor material) from forming on the back side of the wafer.
However, pinholes in the cap material have been found to allow small nodules of silicon to deposit on the back side of the wafer. These nodules remain as irregularities on the surface of the wafer even after the cap material is removed. The pinholes tend to form inherently in many types of dielectric deposition or growth processes. In addition, they are often formed more frequently near the edge of a wafer due to contact by handling apparatus (e.g., tweezers, vacuum pick-up mechanisms, etc.) Furthermore, the pinholes may form more frequently near the edge of a wafer due to a higher concentration of silicon in the epitaxial growth atmosphere, which tends to deplete near the center of the wafer as it sits on the susceptor of the epitaxial deposition chamber. The nodules are especially troublesome around the perimeter of the back side of the wafer, where they can distort the planarity of the wafer when it is clamped in a subsequent process step. For example, lithography operations can be impaired due to lack of planarity caused by the nodules formed around the perimeter of the back side of a wafer.
To prevent the formation of nodules around the perimeter of a wafer, it is known in the art to remove a strip of the cap material around the perimeter of wafer. This allows the subsequent epitaxial deposition process to occur on this strip on the back side (as well as on the front side) of the wafer. Since the epitaxially-deposited material forms a relatively smooth, uniform surface, the wafer can be clamped around its periphery in subsequent lithographic operations without distorting its planarity. However, the prior art techniques for removing the perimeter strip of cap material have not been as rapid or uniform as could be desired. For example, a mask may be applied to the back side that covers the central portion of the wafer, while leaving a perimeter strip exposed. An etching operation can then remove the desired strip of cap material. However, this is a relatively slow process, requiring both a masking and etching operation for each wafer. Another possibility is to insert the wafer edge-wise into a liquid bath of hydrofluoric acid (HF), and roll the wafer until all of the perimeter cap material is etched off by the acid. However, this may not yield as smooth a strip of removed material as is desired, and also may require significant time and operator attention per wafer. Therefore, it is desirable to remove a strip of material from the perimeter of a wafer in a more uniform and/or more cost-effective manner.
Furthermore, a recent trend in the semiconductor art has been to polish the contoured edges of a semiconductor wafer to obtain a rounded and smooth edge. This is desirable to prevent the wafer from abrading the cassettes in which it is stored, or the processing equipment. Such abrasion can produce undesirable microscopic particles that impair the yield of integrated circuits formed on the wafers, especially those formed with sub-micron processing techniques. Prior-art techniques for smoothing the edge of a wafer include polishing with a wheel and polishing slurry, accomplished one wafer at a time. This yields a very smooth edge, but is very time consuming. Alternatively, it is known to roll a batch (e.g., 25) of wafers in liquid polishing acids to smooth the edge, followed by a clean and rinse operation.